For those who do research in the sustainability of cities, there’s a tantalizing puzzle. Per capita, cities are measurably “greener” than other places – their residents have a much smaller ecological footprint. But in our current models, we can explain only about half of that difference, perhaps even less. Like the “dark matter” problem of physics, this “dark greenness” is both somewhat embarrassing, and very intriguing.

To be sure, some of the greenness of cities is not so hard to explain. For example, people drive less in bigger cities, because it’s harder to drive, and because it’s easier to get around without a car. (In fact, driving per person has actually gone down in recent years, even before the economic decline – and evidence suggests it’s because more people are living in compact, walkable cities and suburban neighborhoods.)

Other factors are small in themselves, but do add up: the closer spacing of buildings results in lower transmission losses and pumping energy; there is less embodied energy in roads and other infrastructure, and less operating and maintenance energy; urban residences tend to be more compact and energy-efficient; and more rural and natural areas are preserved, which provide important “ecosystem services.”

But the most intriguing reason may be the one we understand the least: people in cities actually interact and use resources in a more efficient kind of pattern – specifically, a network pattern. When we look at individual factors in isolation, we miss the synergetic effects of this network pattern, which may well explain why we can’t account for the observed magnitude of efficiency.

But there are other models that might help us – especially those that explain the dynamics of networks. In particular, there is a phenomenon in economics that’s known as a “Knowledge Spillover” (also known as a “Jacobs Spillover,” reflecting work on it by the great urban theorist Jane Jacobs). It’s one of the reasons that cities are such powerful economic engines – and very likely one of the reasons we make cities at all. Simply put, it’s the idea that within a city, if you are making x, and I am making y, then our combined knowledge might allow us to make z together – but only if we are physically close enough that our knowledge can “spill over” from one sort of enterprise to another.

In practice, many such “spillovers” gradually connect and reinforce each other, creating a kind of virtuous circle of economic activity, and over time, spawning whole new industries. (Think of the automotive industry centered in Detroit, or the personal computer industry centered around Palo Alto.) This pattern is a classic kind of “network,” familiar to many other disciplines — including those who work with metabolic processes in biology. They have long known that a similar dynamic helps to explain how a body or an ecosystem can have high “metabolic efficiency” – creating new chemicals or structures, and re-using the resources to do so again and again in a sustainable pattern.

It now seems very likely – and a most promising area of research – that something similar happens in cities, and in what we might think of as “resource spillovers.” District energy is an obvious example – I generate electricity, then sell to you the waste heat very cheaply, and we both come out ahead. But the same kind of “metabolic efficiency” can be seen in other, less obvious patterns, like walkable networks. If I am on foot in a neighborhood that offers many of my daily needs, I can easily combine trips, stop to see you, share a meal.

By contrast, if I am in my car, my pattern of movement is much more limited, both by the confining capsule of the car, and by the much larger-scale road pattern. I’m more likely to plug into high-consumption patterns engineered for that more fragmented system (like drive-through restaurants, say). So I am not only burning gas in my car, I’m generating a lot of other forms of high consumption – because I’ve lost the metabolic network efficiency from these “resource spillovers.”

There are many other connections that promote the efficiency of our settlements: the impact on our bodies’ metabolisms and health, the economic vitality of walkable cities, and much more. In each case the lesson is the same: there is power in these self-organzing networks within cities – and there is opportunity in learning to support and enhance them in the way we plan and build.

The corollary is that we have been building a generation of sprawling suburban models – and still are, in too much of the developing world – that have none of this metabolic efficiency, and that rely instead on prodigious inputs of resources to sustain them. In a world of infinite resources, whose consumption had no impact on calamities like resource depletion and climate change, perhaps this would not be a problem. But in our world, it’s a problem of the most serious kind.

Michael Mehaffy is currently doing research on urban sustainability as Sir David Anderson Fellow at the University of Strathclyde in Glasgow. He is executive director of the Portland, Oregon-based Sustasis Foundation.